Image sensors are widely used in digital still cameras, cellular phones, security cameras, as well as in, medical, automobile, and other applications. Complementary metal-oxide-semiconductor (“CMOS”) technology is used to manufacture lower cost image sensors on silicon substrates. In a large number of image sensors, the image sensor commonly includes hundreds, thousand or even millions of light sensor cells or pixels.
Backside illuminated (“BSI”) technology is often used in image sensors to increase the amount of image light that becomes incident on photosensitive elements in pixels of the image sensor. When BSI technology is used, backside accumulation helps create an electric field that directs photo-electrons (generated by incident image light) to a frontside of the image sensor where they can be collected and measured. Without accumulation on the backside the semiconductor substrate, diffusion of the electrons to the back surface may result in loss by recombination, which may degrade the sensitivity of the image sensor. Advantageously, accumulation on the backside of the semiconductor substrate can reduce or impede “dark current” generated from the semiconductor substrate's interface with another layer (e.g. silicon dioxide insulation layer). Reducing “dark current” will improve the signal-to-noise ratio of the image sensor, which importantly, improves the sensitivity of the image sensor. Given that accumulation on the backside of the semiconductor substrate is beneficial, the image sensor industry seeks to generate backside accumulation that will give image sensors improved performance.